This invention relates to a line thermal printer, and more particularly to heat generation control of a line thermal head. A line thermal printer is equipped with a line thermal head having a structure wherein heat generation elements are arranged on a line for effecting thermal printing. Printing density depends on either the quantity of heat generated in the thermal head or the surface temperature of the heat generation elements. When printing is made in line sequence for each line, a thermometer is disposed in the proximity of the line thermal head in order to monitor the temperature change of the thermal head and to keep constant the printing density of each line. Power supplied to the heat generation elements is controlled in accordance with this monitored temperature change so as to keep the surface temperature constant.
In the heat generation control system of the line thermal head in the conventional line thermal printer such as described above, there is a difference between the actual surface temperature of the heat generation element and the detection temperature outputted from the thermometer. Since the heat generated from the heat generation elements is transferred to the thermometer through a radiation plate, a ceramic substrate, etc., of the thermal head, the thermometer is subject to a time delay in response. The detection temperature represents the result of integration of the heat of the thermal head as a whole and cannot follow a quick change below a second unit. For this reason, it has not been possible in the conventional line thermal printer to detect accurately the surface temperature of the heat generation elements of the thermal head on a real time basis and to feed back the detection result for controlling a power supply quantity. Thus, there is the problem that the printing density becomes unstable. About a few seconds' time is necessary, particularly from the start of heating of the thermal head to the response of the thermometer such as a thermistor. Another problem is that the printing density cannot be controlled for a few centimeters in terms of paper feed quantity from the start of printing.